1. The Field of the Invention
The present invention relates generally to optical transmitters. More specifically, the present invention relates to an amplifier circuit for use in a transmitter optical subassembly (“TOSA”) for linearly amplifying a differential input signal.
2. The Relevant Technology
Computing and networking technology have transformed our world. As the amount of information communicated over networks has increased, high speed transmission has become ever more critical. Many high speed data transmission networks rely on optical transceivers and similar devices for facilitating transmission and reception of digital data embodied in the form of optical signals over optical fibers. Optical networks are thus found in a wide variety of high speed applications ranging from as modest as a small Local Area Network (LAN) to as grandiose as the backbone of the Internet.
Typically, data transmission in such networks is implemented by way of an optical transmitter (also referred to as an electro-optical transducer), such as a laser or Light Emitting Diode (LED) mounted on a header within a transmitter optical subassembly (“TOSA”). The electro-optic transducer emits light when current is passed there through, the intensity of the emitted light being a function of the current magnitude through the transducer. Data reception is generally implemented by way of an optical receiver (referred to as an optoelectronic transducer), an example of which is a photodiode, which is generally housed within a receiver optical subassembly (“ROSA”). The optoelectronic transducer receives light and generates a current, the magnitude of the generated current being a function of the intensity of the received light. Both the TOSA and ROSA described above are typically included in an optical transceiver module to enable the transmission and reception of optical signals on behalf of a host device in which the transceiver is operably received.
Various other components are also employed by the optical transceiver (“transceiver”) to aid in the control of the optical transmit and receive components, as well as the processing of various data and other signals. For example, such optical transceivers typically include an electro-optic transducer driver (e.g., referred to as a “laser driver” when used to drive a laser signal) configured to control the operation of the electro-optic transducer in response to various control inputs. The optical transceiver also generally includes an amplifier (e.g., often referred to as a “post-amplifier”) configured to perform various operations with respect to certain parameters of a data signal received by the opto-electronic transducer.
Traditional optical transceivers employ the above-described TOSA that contains a laser in a housing, which housing is configured to couple the optical output of the laser to an optical fiber. The TOSA is attached to a printed circuit board assembly (“PCBA”) included in the optical transceiver that contains circuitry, such as the laser driver to bias and modulate the laser. The circuitry on the PCBA is relatively distant from the TOSA and the laser it contains.
Having the laser driver or other drive circuit located a relatively significant distance away from the laser creates a problem in maintaining good signal integrity. This is especially true for higher bit rates. Specifically, the TOSA and laser driver circuitry are ideally impedance matched to avoid signal reflection and distortion along the transmit path. Generally speaking, this will result in excess power dissipation beyond the minimum required by the laser itself.
Another problem associated with relatively large distances between the laser drive circuitry and the laser is manifested in an increased emission of Electromagnetic Interference (“EMI”) from the transceiver. The amount of EMI generated is proportional to both the drive current provided to the laser by the laser driver and the current loop formed there between.
Various solutions have been proposed or attempted for maintaining the signal fidelity from the laser driver to the laser and/or reducing the drive current and power consumption of a transceiver by incorporating an amplifier positioned near the laser within the TOSA.
FIG. 1 illustrates an amplifier circuit 100 designed to reduce the power dissipated by a transceiver. The circuit 100 is mounted on and grounded to a header 105 and includes an input node for receiving a single-ended signal over a transmission line 110, the single-ended signal being provided to the base terminal of a bipolar transistor 120. A return ground 130 coupled to the emitter terminal of the transistor 120 draws current through the transistor and consequently draws current through an electro-optical transducer 140 coupled to a voltage source 150. The amount of current drawn through the electro-optical transducer 140 depends on the single-ended signal applied at the base terminal of the bipolar transistor 120 via the transmission line 110.
Notwithstanding its ability to reduce laser driver current and therby reducing EMI and overall power consumption, the amplifier circuit 100 shown in FIG. 1 nevertheless suffers from a number of disadvantages. First, maintaining signal fidelity of a single-ended signal is difficult as it requires a high fidelity radio frequency (“RF”) ground to provide a return path for the signal. This requires a very low inductance ground in the signal return path. Moreover, since the RF ground is connected to the header and the header is usually required to be connected to the chassis ground of the transceiver to help dissipate the heat, this can lead to a compliance problem in the system in which the transceiver's chassis is required to be separated from the signal ground. Second, the linear range of the amplifier 120 is limited. When the transistor 120 operates beyond its linear range, waveform shaping of the single-ended received signal is not preserved in the amplified output signal.
FIG. 2 illustrates a differential amplifier and laser circuit 200 designed to maintain signal fidelity. The circuit 200 is mounted on a header 205 and includes two input nodes for receiving differential data signals over differential transmission line 210, a positive signal of the differential signal pair being provided to the base terminal of a first bipolar transistor 220 and a complementary signal being provided to the base terminal of a second bipolar transistor 230. The collector of the second transistor 230 is coupled to an electro-optical transducer 240, and the emitter terminals of both transistors 220 and 240 are coupled to a current source 250. The current source 250 draws current through either the bipolar transistor 220 or the bipolar transistor 230, or through both of the bipolar transistors 220 and 230 in a split manner. The amount of current drawn through the electro-optical transducer 240 depends on the differential data signals applied at the base terminal of the corresponding bipolar transistors 220 and 230.
Despite maintaining signal fidelity via its use of a differentially driven signal, the circuit 200 has numerous disadvantages. First, it dissipates a significant amount of power, half of the power being dissipated through the first transistor 220 without drawing current through the transducer 240. Second, the amplifier circuit 200 is not a linear amplifier, but rather digital. Thus, any waveform shaping performed on the differential signal prior to being provided to the circuit 200 is not preserved when the signal is amplified.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.